The present invention relates to signal encoding, and more particularly to a method and apparatus for encoding and decoding optical power and non-payload data in an optical signal.
In modern communications networks, communications traffic is often carried on optical fibers. A plurality of transmitters may be connected to a first end of a fiber through an optical multiplexer to transmit a corresponding plurality of optical payload signals thereon. Each transmitter includes a semiconductor laser whose output optical intensity is modulated between a first predefined intensity representing a logical xe2x80x9c0xe2x80x9d and a second predefined intensity representing a logical xe2x80x9c1xe2x80x9d to generate an optical payload signal. Using a method called wavelength division multiplexing (WDM), the lasers transmitting on the fiber are chosen such that each laser generates a signal having a unique optical wavelength, permitting a plurality of payload signals to be optically multiplexed onto a single optical fiber without becoming mixed. An optical demultiplexer is connected to an opposite end of the fiber to demultiplex the plurality of optical payload signals onto a corresponding plurality of optical links for receipt by a corresponding plurality of receivers.
To manage such a communications network efficiently, it is necessary to monitor power of the optical signal, to ensure that the signal will be properly propagated through the optical fiber to be detected at a receiver. One method of facilitating optical power measurement is disclosed in U.S. Pat. No. 5,513,029 to Roberts, wherein an optical payload data signal is submodulated with a dither signal encoded with a pseudo random code, and having a submodulation depth which is maintained constant relative to mean optical power of the submodulated optical payload data signal. A pseudo random code is used to broaden the emission spectrum of lasers used in optical transmitters to reduce non-linear optical effects such as Stimulated Brillouin Scattering (SBS).
In addition, to effectively manage an optical system it is desirable to transmit communications and control information between nodes on an optical network. This is typically done by including communications and control information, also referred to as overhead data, in the payload data signal or by submodulating the payload signal with overhead data.
Thus, submodulation of an optical signal has been used for either power monitoring or for transmission of communications and control data. Typically, where submodulation is used for power monitoring, communications and control data is transmitted in payload data and where submodulation is used for payload data, power monitoring is not provided.
Generally, it is not possible to simply add the communications and control data to the pseudo random codes used for power monitoring as this can result in a submodulation depth greater than an allowable limit. Exceeding such a limit may limit the distance the light can travel in the fiber, requiring more optical amplifiers at shorter spacings.
Furthermore, while channel power information is directly measurable when the channels are separate, prior to WDM multiplexing or after demultiplexing, it is difficult to directly measure the power of individual channels of a WDM signal. One existing method of determining channel power of a WDM signal involves optically demultiplexing the WDM signal to retrieve individual optical payload signals, converting the individual optical payload signals into individual electrical signals and then measuring the power of each such electrical signal. However, this method requires the use of a relatively expensive optical demultiplexer and may not, therefore, be economical.
Thus, there is a need for a way to transmit power information and non-payload data in an optical signal without excessive depth of modulation and without interfering with the payload data, while facilitating economical power measurement.
The present invention addresses the above need by providing a method and apparatus for encoding optical power and non-payload data in an optical signal, which involve producing a dither modulating signal having an amplitude indicative of the optical power in the optical signal and having a phase representing non-payload data, and modulating the optical signal with the dither modulating signal.
Such a system facilitates the use of a constant depth of modulation of the optical signal while enabling both optical power and non-payload information to be transmitted, independently of the payload data encoded in the optical signal. Thus, the optical signal need not be demodulated to enable the payload data to be combined with non-payload data, thereby reducing the expense of the optical system.
In one embodiment, a depth of modulation is produced in the optical signal in response to the amplitude of the dither modulating signal and the amplitude of the dither modulating signal may be varied in response to the optical power of the optical signal, to maintain a constant depth of modulation.
The optical mean power of the optical signal may be measured and a representation of a measured depth of modulation of the optical signal may be produced by squaring a representation of the optical signal to produce a tone signal having a tone signal amplitude representing the measured depth of modulation. A digital representation of the tone signal amplitude and a digital representation of the optical mean power may be used by a processor to compute a ratio of modulation depth to mean optical power, and this ratio may be used for controlling a waveform generator to adjust the amplitude of a reference waveform to produce an amplitude adjusted waveform. This amplitude adjusted waveform may be binary phase shift keyed (BPSK) by the non-payload data to produce the dither modulating signal. The light output of a laser may be controlled in response to the dither modulating signal to modulate the optical signal. Thus, instead of encoding the dither signal with a pseudo random code as in the prior art, the dither signal is phase-encoded by non-payload data. The amplitude of the dither signal encodes optical power information while the phase of the dither signal encodes non-payload data. Power and non-payload data are thus encoded in the same dither signal.
In accordance with another aspect of the invention, there is provided an optical signal produced by the method described above.
In accordance with another aspect of the invention, there is provided an apparatus for encoding optical power and non-payload data in an optical signal. The apparatus includes provisions for producing a dither modulating signal having an amplitude indicative of the optical power in the optical signal and having a phase representing the non-payload data and provisions for modulating the optical signal with the dither modulating signal.
In accordance with another aspect of the invention, there is provided an apparatus for encoding optical power and non-payload data in an optical signal. The apparatus includes a waveform generator for producing an amplitude adjusted waveform having an amplitude responsive to the optical power of the optical signal and further includes a binary phase shift keying modulator for binary phase shift keying the amplitude adjusted waveform in response to the non-payload data to produce a dither modulating signal having an amplitude indicative of the optical power in the optical signal and having a phase representing the non-payload data.
Preferably, the waveform generator includes a tone generator for generating a tone signal representing a depth of modulation in the optical signal due to amplitude of the dither modulating signal. The waveform generator may include an optical power signal generator for generating an optical power signal representative of optical mean power of the optical signal.
The apparatus may further include a reference waveform generator for generating a reference waveform of constant amplitude and may include a gain controlled amplifier for amplifying the reference waveform to produce the amplitude adjusted waveform. The gain controlled amplifier may be controlled by a processor circuit which produces a gain control value for controlling the gain controlled amplifier in response to the optical mean power and depth of modulation of the dither modulating signal.
Preferably, the processor circuit is programmed to adjust the gain control value such that the dither modulating signal has a constant modulation depth in the optical signal. The processor circuit is also preferably programmed to compute a ratio of the modulation depth to optical mean power and to adjust the gain control value to maintain the ratio of modulation depth to optical mean power constant.
The apparatus may further include a laser having a bias current control for receiving the dither modulating signal to modulate an optical signal produced by the laser in response to the dither modulating signal. In accordance with another aspect of the invention, there is provided a method of extracting non-payload data from an optical signal modulated with a dither modulating signal carrying the non-payload data by producing an electrical representation of the optical signal, demodulating the electrical representation to extract a binary phase shift keyed signal, and demodulating the binary phase shift keyed signal to obtain the non-payload data.
The method may further include squaring the binary phase shift keyed signal to produce a tone signal having an amplitude representative of the power of the optical signal. The value representing the amplitude of the tone signal may be multiplied by a predefined value to determine the power of the optical signal.
In accordance with a further aspect of the invention, there is provided an apparatus for extracting non-payload data from an optical signal modulated with a dither modulating signal carrying the non-payload data. The apparatus may include an optical to electrical signal converter for converting the optical signal into an electrical signal, a first demodulator for demodulating the electrical signal to extract a binary phase shift keyed (BPSK) signal from the, electrical signal and a BPSK demodulator for demodulating the binary phase shift keyed signal to obtain the non-payload data.
In accordance with another aspect of the invention, there is provided a method and apparatus for measuring optical power in individual optical signals of a composite optical signal. An optical to electrical signal converter converts the composite optical signal into a composite electrical signal, a demodulator demodulates the composite electrical signal to produce a composite dither signal comprised of a plurality of dither modulating signals and a squarer squares the individual dither modulating signals to produce a composite squared signal including a plurality of tone signals representing optical power in respective individual optical signals.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.